• INTRODUCTION

  • In a refinery furnace, heat exchanger is a vital component required for preheating the throughput before it reaches the furnace, thus recovering a significant portion of energy generated in the refinery. Without heat exchangers refining cost will be substantially higher. The throughput is made to flow through a number of heat exchangers arranged as cold pre heat train and hot pre heat train before entering the furnace.
  • It is essential that all heat exchangers must function near optimally in order for the throughput to achieve the desired CIT (coil inlet temperature) at furnace entry.
  • Heat exchangers are prone to internal fouling (tube side) and it is more so when the crude is (bitumen bearing) in nature. This problem will sometimes be so severe that a shut down needs to be taken before the scheduled end of run.
  • It is possible to achieve the above, by making the flow of throughput more turbulent (less laminar). Thus, tube inserts are employed to simultaneously carry out two functions:
  • To enhance turbulence in the throughput flow (increase the Reynold’s Number)
  • Through mechanical action, inhibit the rate of deposition as well as restrict it to a low level
  • PROBLEMS ENCOUNTERED IN THE HEAT EXCHANGERS

  • 1.
    Fouling of heat exchanger tubes
  • 2.
    Reduced general duty
  • 3.
    Reduction in flow rate
  • 4.
    Increase in ∆P
  • 5.
    Reduction in productivity
  • 6.
    Increase in fuel consumption
  • 7.
    Limited service time
  • Tube inserts are used to improve the heat transfer efficiency by cleaning up the existing fouling and prevention of fouling to improve the heat transfer.
  • VARIOUS TYPES OF TUBE INSERTS USED IN HEAT EXCHANGERS

  • SPIRELF
  • TURBOTAL
  • FIXOTAL
  • SPIRELF

  • Spirelf is based on the insertion of flexible metal devices in spiral form in the tubes of the shell and tube heat exchanger. The devices are stretched over the length of the heat exchanger and are held in places by straight wires at each end.
  • The insert stays under tension under the effect of the circulating flow and enters into vibrations both radially and axially. This helps in the breakage of boundary layer in the tube side and prevents formation of deposits.
  • The mechanical effect of the Spirelf reduces fouling in the heat exchanger tubes by 80% and the turbulent action increases the heat transfer rate (internal heat transfer coeff. X 1.8). The spacing of the Spirelf in the tube should be 2.0mm away from the tube surface for better vibration.
  • TURBOTAL

  • Turbotal is based on the insertion of rotating metal devices in helicoidal form into the tubes of the shell and tube type heat exchanger. It is held at the inlet of the tubes by a fixing device allowing the mobile to rotate around its axis by means of the fluid flow.
  • The rotation causes high turbulence in the flow and improves the internal heat transfer coefficient. The boundary layer is continuously renewed, the wall temperature is lowered and fouling is slower.
  • The fixing device of the coil is placed at the outside of the tube. The design of the coil is adapted to the geometry of the tubes and the process conditions. The mechanical effect of the Turbotal reducing fouling build up and turbulent effect achieves an improvement of the heat transfer rate (internal heat transfer coeff. X 1.8). The spacing between the tubes and the Turbotal should be 1.5 - 2.0mm for better rotation.
  • Fixotal consists of a wire coil which is inserted inside every tube with the wire in contact with the inside tube wall. Once in place the device has no possibility of slightest displacement, the device can be easily removed.
    The main effect of the wire is to create turbulence thus decreasing the boundary layer at the wall. This results in increase of the tube side heat transfer resulting in a moderate increase of pressure drop through the tube.
  • FIXOTAL

  • Fixotal consists of a wire coil which is inserted inside every tube with the wire in contact with the inside tube wall. Once in place the device has no possibility of slightest displacement, the device can be easily removed. The main effect of the wire is to create turbulence thus decreasing the boundary layer at the wall. This results in increase of the tube side heat transfer resulting in a moderate increase of pressure drop through the tube
  • KEY ADVANTAGES OF TUBE INSERTS

  • Sustenance of heat duty
  • Reduction of fouling
  • Based on the data of the problematic heat exchangers we can recommend which type of insert would be the best suited and customize the most relevant solution.
  • BENEFITS OF TUBE INSERTS

  • Reduction in fouling
  • Heat transfer improvement
  • Increase in run time
  • Higher outlet temperature
  • Fuel savings at the furnace
  • Reduction in production losses and
  • Savings in maintenance